3.2 Nucleophilic Substitution

Nucleophilic substitutions are common reactions in Organic Chemistry. They evidence how bond polarisation and partial charge can result in reactivity of a functional group. This lesson will introduce you to the different types of common nucleophilic substitution reactions in Organic Chemistry.

Nucleophilic Substitution Reactions

One of the first organic reaction mechanisms you will learn about in a Chemistry degree, and are perhaps already familiar with, is nucleophilic substitution. In principle, a nucleophile is any atom or molecule that is attracted to positive charges and atoms which are electron-poor. The name comes from the Latin nucleus (the central part - i.e. the positively charged part of an atom) and the Greek word philus (beloved). Therefore, nucleophile literally means core-loving, with core meaning positive charge.

Conversely, an electrophile is any atom or molecule that is attracted to a negative charge and atoms that are electron-rich. If you think back to the polarisation of bonds you can begin to see how some sites of functional groupsAny group of atoms in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest of the molecule's composition. are electron rich/poor and therefore can act as nucleophiles or electrophiles.

Typical functional groups that are susceptible to nucleophilic attack are carbons bound to electronegative atoms like oxygen (for example in carbonyls) or halogens. The electronegative atoms attract so many electrons away from the carbon so that it becomes partially positively charged, making it an electrophile that is susceptible to attack by a nucleophile.

In a nucleophilic substitution, a nucleophile can be thought of as ‘attacking’ the electrophile, forming a new covalent bond between the nucleophile and electrophile. For this to take place, the electrophile must break one of its original bonds to accommodate the new bond with the nucleophile. The breaking of this bond results in the loss of a group from the molecule. This group is called the leaving groupFunctional groups which leave during the reaction. Good leaving groups are often groups or atoms which happily exist as negatively charged species (eq. halogens). The rule of thumb is the more basic a leaving group is, the worse it is.. The two common ways in which a nucleophilic substitution can occur are in a \(\ce{S_N 1}\) or \(\ce{S_N 2}\) mechanism.